This invention relates to a gas separating system and a gas recovery system. More particularly, it relates to a pressure swing type gas separating system (hereinafter sometimes called a PSA system) used for refining and recovering gases.
A conventional PSA system is shown in FIG. 21. An absorber 302 is filled with adsorbent 301. From one end of the absorber 302, a pipe 307 is connected to an untreated gas introduction valve 303 and an evacuation valve 305, whereas from the other end of the absorber 302, a pipe 308 is connected to a treated gas discharge valve 304 and a refining gas introduction valve 306.
In the adsorption process of this conventional PSA system, the evacuation valve 305 and the refining gas introduction valve 306 are closed and the untreated gas introduction valve 303 and treated gas discharge valve 304 are opened, whereby untreated gas is allowed to flow in the absorber 302. In the absorber 302, a particular component in the gas (for example, CO.sub.2, NH.sub.3, O.sub.2, etc.) is adsorbed onto the adsorbent 301; as a result, treated gas which has passed through the absorber 302 without being adsorbed onto the adsorbent 301 can be obtained.
Next, in the desorption process, the valve operation reverse to the adsorption process is performed. Then, the absorber 302 is depressurized by a not illustrated vacuum pump, and a small amount of refining gas is allowed to flow into the absorber 302. Thus, the particular component adsorbed onto the adsorbent 301 is desorbed and discharged through the evacuation valve 305.
By repeating the above cycle, a particular gas component in the untreated gas is recovered, or a particular gas component is refined.
The above-described conventional PSA system has some disadvantages as described below because the adsorption and desorption processes are repeated and these processes are performed by an intermittent operation carried out by means of many valves. The disadvantages are as follows:
(1) Large amounts of adsorbent are required because the time of one cycle is as long as several minutes. PA0 (2) A gas supplying means for untreated gas and an evacuating means (vacuum pump) are continuous rotating turbo machines, which do not match the intermittent operation, offering low efficiency. Specifically, the evacuating means operates wastefully in the adsorption process, whereas the gas supplying means does in the desorption process. PA0 (3) To increase the capacity of the PSA system, increased amounts of absorbent and many large-sized valves as well as the opening/closing control are required, resulting in higher cost. PA0 (4) The impact noise accompanying the opening/closing of valves and the noise caused by sudden change of gas pressure in the circuit are high. PA0 (1) The cycle time can be shortened, and the amount of adsorbent with which the adsorbing portions are filled can be decreased. PA0 (2) The power loss is low and the gas separation efficiency can be increased because the gas supplying means and the evacuating means have no wasteful waiting time. PA0 (3) A system of high capacity and low cost can be easily provided because a rotating valve plate is used in place of the conventional on-off valve. PA0 (4) The change in gas pressure in the system is smaller than that in the conventional system, resulting in lower noise. PA0 (1) Since the valve plate performs oscillation motion and the openings therein have a small radius of rotation, the sliding speed of valve plate in relation to the first and second manifolds can be decreased, thereby the required driving power being reduced. PA0 (2) The openings in the first and second manifolds and the valve plate are not necessarily arranged radially, which provides larger degree of freedom in arrangement, thereby the system being made compact. PA0 (3) The cycle time can be shortened, and the amount of adsorbent with which the adsorbing portions are filled can be decreased. PA0 (4) The power loss is low and the gas separation efficiency can be increased because the gas supplying means and the evacuating means have no wasteful waiting time. PA0 (5) A system of high capacity and low cost can be easily provided because oscillating valve plate is used in place of the conventional on-off valve. PA0 (6) There is no need for using large seal members because the seal can be installed independently for each adsorbing portion or for each opening of the first manifold on the side of adsorbing portion.
Also, a rotary gas recovery system shown in FIGS. 22 through 25 has so far been used.
In FIG. 22, 401 denotes an upper fixed seal plate, 402 denotes a lower fixed seal plate, 403 and 404 denote bearings installed to the seal plates, 405 denotes a rotating shaft which is mounted by the bearings for rotation, 406 denotes a key mounted on the rotating shaft, 407 denotes a rotary adsorbing/desorbing device which is fixedly secured to the rotating shaft 405 by the keys 406 and rotates between the upper fixed seal plate 401 and the lower fixed seal plate 402, 408 denotes a material gas supply port installed in the upper fixed seal plate 401, 409 denotes a particular gas discharge port installed in the upper fixed seal plate 401, 410 denotes a product gas discharge port installed in the lower fixed seal plate 402, 411 denotes a small hole for supplying purge gas which is installed in the lower fixed seal plate, 412 denotes a plurality of compartments extending axially through the rotary adsorbing/desorbing device, 413 denotes an adsorbent put into each compartment, 414 denotes a gas compressor for feeding the material gas under pressure through the material gas supply port 408, 415 denotes a vacuum pump which is connected to the particular gas discharge port and depressurizes and sucks the particular gas for discharge and recovery, 416 denotes a seal surface between the upper fixed seal plate 401 and the rotary adsorbing/desorbing device 407, and 417 denotes a seal surface between the lower fixed seal plate 402 and the rotary adsorbing/desorbing device 407.
In the above-described system, the material gas is supplied by the gas compressor 414 to the compartments 412 of the rotary adsorbing/desorbing device through the material gas supply port 408. In the compartments 412, a particular gas is adsorbed onto the adsorbent 413 in the compartment 412, and the gas which has not been adsorbed, that is to say, the product gas is discharged through the product gas discharge port. By the rotation of the adsorbing/desorbing device 407, when the compartment which is filled with the adsorbent onto which a particular gas has been adsorbed comes into communication with the particular gas discharge port 409, the particular gas is removed from the adsorbent 413 by the evacuating action of the vacuum pump 415 and discharged for recovery. In this process, purge gas is fed through the small hole for supplying purge gas in the lower fixed seal plate 402. Thus, the product gas is produced and the particular gas is recovered continuously by the rotation of the adsorbing/desorbing device 407.
In the above-described system, the adsorbing/desorbing device 407 rotates and the upper and lower seal plates 401, 402 are fixed. Therefore, the mating surfaces cannot be connected completely, and seal surfaces with a gap are provided, so that the material gas leaks from the seal surface 416. In recovering the particular gas, external gas also flows into the particular gas discharge port 409 through the seal surface 416. The leaked gas poses no problem of deteriorated system performance if it accounts for 1% or less of the supplied gas. However, when the system capacity is increased, the manufacturing accuracy of seal plates 401, 402 decreases, and hence the gaps of seal surfaces 416, 417 increase. As a result, the increase in leaked gas deteriorates the system performance. Since the amount of leaked gas is proportional to the third power of the gap, the effect of gap is very large. In addition, the wear of seal surface increases the gap, so that the system performance deteriorates with time. To overcome this problem, it is necessary to carry out maintenance frequently, which increases the running cost of the system.